1. Field of the Invention
The present invention relates to a power supply unit, high-frequency circuit system and heater voltage control method, preferable for supplying heater voltage to an electron tube.
2. Description of the Related Art
Traveling-wave tubes, electron guns and the like are electron tubes for use in performing amplification, oscillation etc. of high-frequency signals by an interaction process between a beam of electrons emitted from an electron gun and a high-frequency circuit.
FIG. 1 is an example of a high-frequency circuit system of a background art.
As shown in FIG. 1, traveling-wave tube 1 includes, for example, electron gun 10 for emitting a beam of electrons, helix 20 as a high-frequency circuit for causing an interaction between electron beam 50 emitted from electron gun 10 and a high-frequency signal (microwave), collector electrode 30 for capturing electron beam 50 traveling inside helix 20 and anode electrode 40 for extracting electrons from electron gun 10 and leading electron beam 50 emitted from electron gun 10 into helix 20.
Electron gun 10 includes cathode electrode 11 for emitting electrons, Wehnelt electrode 13 for converging the electrons emitted from cathode electrode 11 and heater 12 for supplying thermal energy for causing emission of thermal electrons from cathode electrode 11.
Electron beam 50 emitted from electron gun 10 is accelerated by the potential difference between cathode electrode 11 and helix 20 and lead into helix 20 so that it travels therethrough while interacting with a high-frequency signal input to helix 20. The electron beam propagating inside helix 20 is captured by collector electrode 30. As a result of this process, a high-frequency signal that has been amplified by interaction with electron beam 50 is output from helix 20.
As shown in FIG. 1, a negative DC voltage based on the potential of helix 20 is supplied as helix voltage EheI to cathode electrode 11 while a positive DC voltage based on the H/K potential of cathode electrode 11 is supplied as collector voltage EcoI to collector electrode 30. A negative DC voltage based on the H/K potential of cathode electrode 11 is supplied as heater voltage H to heater 12.
Helix 20 is grounded by connection to the case of traveling-wave tube 1. Anode electrode 40 is connected to, for example helix 20 so that it is set at the same potential as helix 20. In some cases, traveling-wave tube 1 may take a configuration in which anode electrode 40 is not connected to helix 20. In this case, a positive DC voltage based on the H/K potential of cathode 11 is supplied as anode voltage Ea to anode electrode 40.
Helix voltage EheI, collector voltage EcoI, anode voltage Ea and heater voltage H are generated using for example, inverters, which are well known to covert a DC voltage to an AC voltage, transformers, rectifying circuits and capacitors for rectification and the like. FIG. 1 shows a configurational example of traveling-wave tube 1 including a single collector electrode 30. However, traveling-wave tube 1 may include two collector electrodes or three or more collector electrodes.
In a case where the thus constructed traveling-wave tube 1 is operated, it is necessary to supply heater voltage H first to pre-heat cathode electrode 11 (about 3 to 5 minutes) and then supply helix voltage EheI, anode voltage Ea and collector voltage EcoI after completion of preheating. Accordingly, there are many cases in the power supply unit for traveling-wave tube 1, in which the heater power supply circuit for generating heater voltage H is provided independently from the high-voltage power supply circuit, for generating helix voltage EheI, anode voltage Ea and collector voltage EcoI.
FIG. 2 shows a configuration of a power supply unit in the background art for supplying the heater voltage shown in FIG. 1.
As shown in FIG. 2, the power supply unit of the background art includes transformer 300, DC voltage source 311, inverter 310 for converting the DC is voltage output from DC voltage source 311 into an AC voltage to be supplied to the primary coil of transformer 300, rectifying circuit 330 for rectifying the AC voltage output from the secondary coil of transformer 300 to supply a DC voltage to heater 12, timer circuit 320 to be used for measurement of preheating time, and control circuit 340 that controls the operation of inverter 310 and outputs an HV ON/OFF signal as a control signal for enabling the output of helix voltage EheI, anode voltage Ea and collector voltage EcoI after a lapse of a predetermined preheating time set up at the time that power is activated based on timer circuit 320.
Inverter 310 includes transistors Q21 and Q22 for converting the DC voltage output from DC voltage source 311 into an AC voltage and driver circuit 312 for turning on and off transistors Q21 and Q22 alternately.
Rectifying circuit 330 includes a full-wave rectifying circuit made of two diodes, for example and rectifies the AC voltage output from the secondary coil of transformer 300 to output as heater voltage H a negative DC voltage based on the H/K potential of cathode electrode 11 (not shown).
In the configuration for controlling the preheating time by control circuit 340 based on timer circuit 320 shown in FIG. 2, usually, the preheating time is set at a sufficiently large value so that the temperature of cathode electrode 11, not illustrated, rises high enough and traveling-wave tube 1 operates stably. For this reason, the preheating time tends to be longer than needed, hence it takes a long time from the activation of the power supply until operation of traveling-wave tube 1 starts.
As a method of shortening the preheating time, there is a configuration disclosed in Japanese Patent Application Laid-open No. 06-310045 in which heater voltage H is set higher than normal voltage at the time of power activation and then heater voltage H is changed to normal voltage when a temperature sensor detects that cathode electrode 11 has reached a predetermined temperature.
Now, a case will be considered where a power failure occurred in the DC voltage source during normal operation in the power supply unit of the background art shown in FIG. 2.
In the power supply unit of the background art, if a power failure occurred in the DC voltage source during normal operation and recovers from the power failure, the timing for sending out an HV ON/OFF signal is also controlled by the control circuit.
For restoration of the power supply after a power failure, the power supply unit of the background art usually adopts either the method of sending out an HV ON/OFF signal after a lapse of the predetermined preheating time in the same manner as is done at the time of power activation or one of the methods of changing the preheating time in accordance with the time (power failure time) at which the power has been cut off. As the method of changing the preheating time in accordance with the power failure time, it is typical to set the preheating time to be equal to the time of power failure. In this case, however, the maximum of the preheating time is as long as about 3 to 5 minutes, which is the preheating time at the time of power activation.
However, according to the method of sending out an HV ON/OFF signal after a lapse of the predetermined preheating time, in order to heat cathode electrode 11 to a high enough temperature, a preheating time that is as long as about 3 to 5 minutes, as stated above, is secured even if a so-called instantaneous power failure of some seconds has occurred. Accordingly, this method entails the problem that the time for restarting the operation of the traveling-wave tube becomes wastefully long.
On the other hand, according to the method of setting the preheating time to be as long as the time of the power failure, an effective way is to make the length of the power failure longer than a certain period (longer than 25 seconds in the example shown in FIG. 3) as shown in the graph of FIG. 3, for example. However, when the period of power failure is short (instantaneous), it may happen that the temperature of the cathode electrode cannot reach the operable temperature. This problem is attributed to the fact that since there are the parts that support the cathode electrode and the parts that are arranged close to the cathode electrode, it takes time for the cathode electrode and these parts to reach a state of thermal equilibrium. If the traveling-wave tube is operated by supplying helix voltage EheI, anode voltage Ea and collector voltage EcoI in a state in which the temperature of the cathode electrode has not been elevated high enough, the operation of the traveling-wave tube becomes unstable. FIG. 3 shows how the temperature of the cathode electrode behaves when all the power supply voltages of the traveling-wave tube are turned OFF from normal operation and the state afterwards in which the fixed voltage for normal operation has been applied to the heater for a period of time which is as long as the time in which the power supply voltages have been in an OFF condition. Further, in FIG. 3 the temperature characteristics of the cathode electrode, when supplying power voltages is halted at timings of 5, 10, 15, 20, 25, 30, 40, 50 and 60 seconds, are depicted in the order from the left. As shown in the graph of FIG. 3, it is understood that when the power failure time is relatively short such as 5 seconds, 10 seconds, 15 seconds etc., the temperature of the cathode electrode can not reach the minimum operable temperature when the duration for preheating the cathode electrode is equal to the duration of the power failure.